docs: def clear(self, *objs):
          """
          Clear the third relationship table, but not the ModelA or ModelB
          """
          if objs:
              keys = get_objs_columns(objs, self.realfieldb)
              self.do_(self.table.delete(self.get_default_condition() & (self.table.c[self.fieldb].in_(keys))))
          else:
              self.do_(self.table.delete(self.get_default_condition()))
          #cache [] to _STORED_attr_name
          setattr(self.instance, self.store_key, Lazy)